# Generic optical excitations of correlated systems: $\pi$-tons

## TL;DR

This paper introduces $$-tons, a new type of polariton arising from antiferromagnetic or charge density wave fluctuations in strongly correlated systems, significantly affecting optical conductivity across various models.

## Contribution

It identifies and characterizes $$-tons as a novel polariton type in correlated materials, extending the understanding of light-matter interactions beyond excitons.

## Key findings

- $$-tons dominate the vertex correction to optical conductivity.
- They are prevalent in multiple correlated models, both insulating and metallic.
- $$-tons influence optical properties in strongly correlated systems.

## Abstract

The interaction of light with solids gives rise to new bosonic quasiparticles, with the exciton being---undoubtedly---the most famous of these polaritons. While excitons are the generic polaritons of semiconductors, we show that for strongly correlated systems another polariton is prevalent---originating from the dominant antiferromagnetic or charge density wave fluctuations in these systems. As these are usually associated with a wave vector ${\mathbf k}= (\pi,\pi,\ldots)$ or close to it, we propose to call the derived polaritons $\pi$-tons. These $\pi$-tons yield the leading vertex correction to the optical conductivity in all correlated models studied: the Hubbard, the extended Hubbard model, the Falicov-Kimball, and the Pariser-Parr-Pople model, both in the insulating and in the metallic phase.